A typical wire cut electrical-discharge machining apparatus uses a thin wire of about 0.05 to 0.35 mm in diameter as an electrode tool. The wire is disposed under a predetermined tension between a pair of guide members in such a manner that the portion of the wire in the working zone can be renewed by axial displacement while maintaining the wire under tension.
A contour may be cut in the workpiece to be machined by moving the workpiece with respect to the wire electrode and in a direction which is generally orthogonal to the axis of the wire electrode, while maintaining a minute gap between the wire and the workpiece and under conditions wherein a working fluid is introduced into the gap between the wire and work piece. Contour machining is carried out by impressing voltage pulses across the gap between the wire and the workpiece and moving the workpiece with respect to the wire electrode in order to establish a work feed in the desired machining direction.
However, unless suitable operating conditions are maintained during machining, wire breakage is apt to interrupt production.
Recently, WC-EDM's have been available which have included a self-recovery means for dealing with wire electrode breakage. More specifically, WC-EDM's have been provided with functions for automatically inserting and joining the wire electrode ends. However, these arrangements do not necessarily assure 100% effective or reliable automatic wire electrode insertion and joining. Further, even if a repair is appropriately achieved, the rethreading function not only wastes time but also generally impairs machining accuracy, finish, and the like. Hence it is preferable in the first instance to avoid the causes of wire breakage.
When wire cut electrical-discharge machining, a number of problems which tend to cause wire breakage may be encountered. It is usually impossible to carry out high speed machining with accuracy and high efficiency unless high-load working conditions, and in particular electrical conditions such as voltage pulse width, off time duration and amplitude of the discharge current, and machining feed control, are established and controlled. However, when machining under high load conditions, there is a higher risk of wire breakage. In addition, for precision machining, other conditions, such as high wire tension and the like, are required which exacerbate the danger of wire breakage.
Therefore, some recent WC-EDM's have included devices to carry out various detection and control strategies in order to prevent wire electrode breakage. Further in the event of wire electrode breakage, some recent WC-EDM's have included devices to quickly detect the break and, thereafter, execute a control function which at least temporary arrests the supply of machining voltage pulses or power, or arrests the travelling (renewal) of the wire electrode, or temporally arrests the machining action including cutting off the work fluid supply, machining feed and the like. This is done to prevent malfunctions and the consequences of such abnormal failures on the workpiece, the working fluid nozzle, the supply and travel systems for the wire electrode and the like in the event of wire electrode breakage.
However, the foregoing broken-wire detection systems are not necessarily as quick as desired. Accordingly, it has been difficult to avoid obstacles to machining progress and accuracy due to excessive detection and control, and problems involving different failures which result from delays in adaptive control.
There are various type of wire electrode broken-wire detectors or broken-wire detecting control devices already available for conventional wire cut electrical-discharge machining apparatus. However, most of these devices are of the type wherein discrimination is based on detection on the outboard side of the supply side guide member, i.e., between the supply side guide member and the supply reel and at a location such as along the wire feed path along or at structures positioned between the supply reel and the supply side guide member over which the wire is strung, and/or on the outboard side of the take up side guide member and used wire recovery member. Typical devices use sensors with contact(s) such as limit switches and photodiode sensors, or operate to detect the tension and/or change in travelling speed of the wire electrode.
However, as most wire breakage occurs proximate the workpiece and between the upper and lower guide members, the indirect detection at the outboard sides of the interspace between the guide members has resulted in a problem in that accurate detection by the various sensors is delayed.
As disclosed in JP-A-53-68496 for example, it is known to arrange a current source to supply a minute current, which is not directly related to the working current, to the wire electrode from feed members disposed above and below the work table on which the workpiece is fixed, and to use a current detector to detect any fluctuation of the minute current. This permits detection of a break in the electrode wire by detecting when the minute current drops to zero.
However, the above broken-wire detector is so arranged that minute currents are by necessity supplied from the current source to the feed members above and below the workpiece. Therefore, it exhibits the disadvantage of being in contact with the machining power supply by way of the upper and lower feed members and the workpiece. The minute current flows from the current source through the incoming line for machining power at the time of breaking of the wire electrode. This inhibits positive detection of a wire break condition.
In addition, with this type of broken-wire detector, in some cases it is possible that for a short time after a wire break occurs, both free end pieces of the wire in the vicinity of the break will make repeated contact with locations on the workpiece and with other electrically conductive material adjacent to the wire electrode guide path, thus preventing the minute currents from being immediately and completely cut-off. This often results in delayed detection of a wire break.
In addition, as above described, the present invention relates to an improvement in detection of wear of one or both of the feed members. This is because, in the case where wire breakage detection is carried out using a method where detection is accomplished through the electrical power supply elements using the electrical power supply circuit, it can by-pass the above mentioned wear detection means.
By way of example, JP-A-60-108226 discloses a wire cut electrical discharge machine comprising a plurality of feed members which are provided with diodes which causes electrical current to flow in the same direction. These feed members are connected to a power source, of either direct or alternating current, of a predetermined magnitude. The electricity from the power source is supplied by way of an impedance element and wire breakage is detected by comparing the voltage across the ends thereof with a reference value.
In JP-A-63-109915, an electrical-discharge machine is disclosed which includes a circuit which supplies electric energy to the workpiece from the feed members, a wire electrode break detecting circuit to detect a break of the wire electrode, means to detect the current flowing from the electrical-discharge machining power circuit into the wire electrode break detecting circuit when wear of the feed members occurs, and means to detect the machining speed. With this arrangement, the values detected by the current detecting means and machining speed detecting means are compared with respective standard values and the time for replacement appropriately indicated.
However, both of these wire electrode break detecting means suffer from delayed detection in that both working current and any separate current, both flow through the wire electrode. Further, as the feed members wear, the electrode break detecting means, and similar types of detection means and arrangements which are associated with the establishment of electrical contact by the electrical feed members, suffer impaired detection accuracy.
In addition, in JP-A-4-129617, an electrical-discharge machine is disclosed wherein the existence of an abnormality is determined on the basis of the detected voltage, and includes means for detecting the voltage between the wire electrode and the feed members. However, in order for the voltage at a location immediately adjacent the feed members to be detected, it is necessary to provide a contact which is separate from the feed members and used only for the detection of the wire electrode voltage. This not only leads to a complicated and costly arrangement, it also leads to a situation wherein an abnormality due to detecting conditions at the contacts may occur. This of course renders accurate detection unlikely.